The present invention relates to a process for producing poly (p-phenylenesulfide), more particularly, to a process for producing poly (p-phenylenesulfide) of increased molecular weight.
Poly (p-phenylenesulfide) has high resistance to heat and chemicals and use of it in electrical and electronic parts, as well as in automotive parts is drawing attention of researchers. This polymer can be injection-molded, extrusion-molded or otherwise molded into various shaped articles including films, sheets and fibers, and the resulting shaped articles find extensive use in applications where resistance to heat and chemicals is required.
One conventional method for producing poly (p-phenylenesulfide) consists of reacting a dihalo aromatic compound with an alkali metal sulfide such as sodium sulfide in a polar aprotic solvent (see Japanese Patent Publication No. 45-3368). However, the polymer produced by this method is too low in molecular weight to be usable in molding applications including injection molding. In order for this low-molecular weight polymer to be used in shaping and processing applications, it is conventionally crosslinked by thermal oxidation to increase its molecular weight. However, even this polymer having increased molecular weight has low adaptability for extrusion, probably due to the high degree of crosslinking and branching and substantial difficulty is involved in shaping it into films or fibers.
In an attempt to solve this problem, methods have been proposed for obtaining poly (p-phenylenesulfide) of increased molecular weight by polymerization reaction. A typical example of this approach is described in Japanese Patent Publication No. 52-12240 and consists of performing the intended polymerization reaction in the presence of R-COOM (R is a hydrocarbyl group and M is an alkali metal) which is used as a polymerization aid. The polymer of increased molecular weight obtained by this method has high adaptability for extrusion molding and can effectively be formed into films, fibers and other shaped articles.
A problem with this method, however, is that only expensive lithium salts used as polymerization aids will exhibit marked effectiveness in providing increased molecular weight and hence, the production cost is increased to a commercially unfeasible level. On the other hand, inexpensive sodium salts are inefficient to provide an intended increase in molecular weight unless a crosslinking agent such as a polyhalo aromatic compound containing at least 3 halogens in one molecule is added. However, not only does this intricate the operations in production process but also the resulting polymer is prone to gelation.